1. Field of the Invention
The present invention relates to a photosensor consisting of a light emitting device and a light receiving device and, more particularly, to a photosensor used in a density sensor unit of an image recording apparatus.
2. Related Background Art
An arrangement in which light from the front surface of a light emitting device 1 is irradiated on a certain medium 10 and the reflected light from the medium is detected by a light receiving device 9 is conventionally known as a photosensor. In this arrangement, the directivity characteristic of the light from the front surface of the light emitting device is generally as shown in FIG. 1. Also, a model in which side beams from a light emitting device 1 are received and monitored by a light receiving device 2 is as shown in FIG. 2. In a conventional photosensor, the light emitting device 1 and the light receiving device 2 are often arranged in positions as shown in FIG. 2 by taking account of the influence of limitations on design and mechanical positional accuracy.
Unfortunately, when the side beams from the light emitting device 1 are monitored by the light receiving device 2 as described above, the light emitting device 1 has two point light sources, i.e., a chip and the top of a resin mold encapsulating the chip when viewed from the light receiving device 2. Accordingly, the emission outputs and the directivity characteristics of these two point light sources vary due to factors such as a temperature rise in the light emitting device chip when the chip is powered, a change in the surrounding environment, and deterioration with time.
FIG. 3 shows an example of a circuit model which causes a light emitting device 1 to emit light and light receiving devices 2 and 9 to receive the light and output electrical signals. Referring to FIG. 3, this circuit model comprises the light emitting device 1, the light receiving device 2 for receiving side beams of the light from the light emitting device 1, the light receiving device 9 for receiving front beams of the light from the light emitting device 1, a comparator 12, a reference voltage terminal 11 for supplying a reference voltage to the comparator 12, a resistor 13 for regulating the power supply amount to the light emitting device, amplifiers 14 and 17, current-voltage converting resistors 15 and 18, a side beam output voltage terminal 16 used when the side beams are converted into an electrical signal, a front beam output voltage terminal 19 used when the front beams are converted into an electrical signal, a light emission amount controller 20 for controlling the light emitting device 1 with a constant current in accordance with a reference voltage Vi, a side beam receiving unit 21, and a front beam receiving unit 22.
FIG. 4 shows variations in the side beam with the power supply amount when the light emitting device 1 is made to emit light by a constant current in the circuit model with the arrangement as shown in FIG. 3. Each parameter is obtained by varying the power supply amount by varying the reference voltage Vi. FIG. 5 shows variations in the front beam with the power supply amount when the light emitting device 1 is made to emit light by a constant current. FIG. 6 shows variations in the ratio of the side beam to the front beam when the light emitting device is made to emit light by a constant current.
As shown in FIGS. 4 to 6, the side beam tends to increase with time and the front beam tends to decrease with time upon the supply of a certain constant current. Consequently, the ratio of the side beam to the front beam is largely corrupted with time. Also, the larger the power supply amount, the larger the side beam, the front beam, and the ratio of the side beam to the front beam is further corrupted.